In article ,
Jim Yanik wrote:

http://www.repeater-builder.com/motorola/genesis/nimh-charger-mod.html
These simplistic chargers are horrible and will kill any battery fed
to it in short order. There are some similar Kenwood chargers with
the same problem. They tend to overcharge, even after the battery
pack gets over-heated. I've modified a few of these with good results
as the author describes.




I've read that charge terminating NiMH by temperature rise is not good or
proper,that it has to be done by voltage monitoring.
By the time the NiMH cells heat up,the damage has been done.

I believe that they're both aspects of the same phenomenon... that is,
the change in the cell's terminal voltage is the result of the rise in
temperature, which itself results from the fact that the energy being
pumped in is now being converted to heat rather than to
electrochemical potential.

In effect, sensing the terminal voltage and shutting down when it
ceases to rise is a faster way of detecting the rise in the cell's
internal temperature. You don't have to wait for the heat to "soak
out" of the cell and warm up the thermistor.

The recommendations I've read suggest that the best shutdown approach
for NiMH cells is via zero-delta-V measurement, with a temperature
sensor as a secondary, and a timer as a last-ditch shutoff.

The temperature sensor is best used to detect "sudden rise in
temperature" rather than "specific temperature threshold". This makes
the detection less sensitive to variations in the room temperature,
and also to the sort of variation in thermistor values written about
in the document Jeff pointed us to. This is probably best done with a
microcontroller or a dedicated charge-control IC that can perform
time-based measurement analysis.

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
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